## Stuck very hard

Advanced methods and approaches for solving Sudoku puzzles

### Stuck very hard

I have three tactics for solving these puzzles but this 'very hard' one, from the sudoku program, seems to be demanding something special.

I've used the obvious checking for consistency in columns, rows, and boxes..
and then used pairs to remove from the same row or column
..eg r6c[6,8] implies r6c[2,4,5] can't be 6 or 8
and then used limited groups of three cells to remove impossible digits
..eg r8c[6,8,9] to remove digit 1 from r8c[1,2,5]

(I can't see 'x-wings' but then haven't used them before)

Any suggestions how to progress this with logic alone?.. that it's a sudoku program problem, I'm thinking suggests it can be solved with logic..
davidpbrown

Posts: 7
Joined: 08 May 2005

I have no idea - sorry - but I'd like to know how you managed to post the puzzle in that format and that big, and can you print it out that big (given the printer/paper constraints)?

Thanks

Luna
lunababy_moonchild

Posts: 659
Joined: 23 March 2005

Yes, nice grid.

Assuming your pencil marks are correct, you have what is known as an X-wing in cols 1 and 5. There are lots of explanations dotted around this site if you don't know what this means...
Guest

I'm still working on it, but here is one number you can eliminate:

The only place for a 4 in column 1 is in box7, so we know that all 4's in box7 can be eliminated (except the ones in column 1)
Animator

Posts: 469
Joined: 08 April 2005

Here's a much better hint:

Box 5 can only have a 4 in column 4,

this means that you can remove the 4 from the list of possible candidates from r9c4, this should allow you to fill in a number
Animator

Posts: 469
Joined: 08 April 2005

Many thanks, Animator.. very quick reply! I should have seen that - I think I've been looking at it too long.

Luna, the grid is a snapshot of the screen, hosted on my website, and then posted using the insert image option. Easy if you've got a snapshot tool (print screen might work), a photo editor to trim the image and a website that allows external image requests.. and then a forum that allows posting images - some don't.

I expect it can print but I've no printer here. The sudoku.com program (trail for a month) it naturally this big and does have a print option.

I'll see if I can't solve it now..
davidpbrown

Posts: 7
Joined: 08 May 2005

Hello. I am new too to Sudoku and find them absolutely fabulous I only started today They make you think really hard about the solution and now I can't wait until tomorows Times for another one I also bought the new book out which has 100 puzzles And again I am now hooked Whoever created them is a genius Love xXx ChilledChick xXx
ChilledChick

Posts: 6
Joined: 08 May 2005

Just solved it.

Your first post their, Animator, was an extension of my doing the same in the row/column.. I hadn't thought of using it in the box itself.. so many thanks for that.
davidpbrown

Posts: 7
Joined: 08 May 2005

Hello. Have you got the new Sudoku puzzle book thats out? It's from the Times and has 100 puzzles in it I have only just learnt how to do these today so I am on the easy one Hopefully I should get better as time goes on
ChilledChick

Posts: 6
Joined: 08 May 2005

Hi ChilledChick,

No, I've not got the Times book.. from the little I've read it doesn't have 'very hard' = 'super fiendish' ones.. just the easy ones ;)

The sudoku.com program appears to have a large set of very hard puzzles.. which are the ones interesting me at the moment.
davidpbrown

Posts: 7
Joined: 08 May 2005

It has Easy, Mild, Difficult and Feindish puzzles.
causbrite

Posts: 20
Joined: 29 March 2005

Indeed.. and from what I read ?here the Times expect for one early problem are only upto sudoku.com program's 'hard' puzzles.

Someone here suggested, for instance, that you wouldn't need x-wings on Times problems.

The program suggests
"If you like solving Sudoku puzzles by guessing or trial-and-error, you might not notice much difference between Hard and Very Hard puzzles. However, you will certainly notice the difference if you like to prove every step of the solution with logic."

(That said I think it might be harder to solve on paper as there wouldn't be enough room for clear pencil marks.)

===========
Edit: for ref. the quote was from
Pappocom post on x wings
Last edited by davidpbrown on Sun May 08, 2005 10:29 am, edited 1 time in total.
davidpbrown

Posts: 7
Joined: 08 May 2005

Oh ok Well I just started doing them today so they are challenging enough Though once I'm better at them i suspect I will move onto something harder than the Times fiendish ones
ChilledChick

Posts: 6
Joined: 08 May 2005

Hi, ChillledChick, you might be interested to know that Pappocom has a book - details elsewhere on this very website - of 100 puzzles out too. They go from Easy to Fiendish, which is hard enough for me!

Glad you enjoy the game though.

Luna
lunababy_moonchild

Posts: 659
Joined: 23 March 2005

davidpbrown wrote:Hi ChilledChick,

No, I've not got the Times book.. from the little I've read it doesn't have 'very hard' = 'super fiendish' ones.. just the easy ones ;)

The sudoku.com program appears to have a large set of very hard puzzles.. which are the ones interesting me at the moment.

The program has an infinite number of puzzles at all levels, it generates them as required. I tend to print them off four or six to a page and then transcribe them to a big Excel grid I've made. That's my only issue with the program, the lack of printing customisation.

It's a bargin

Duncan

**No Trial, some errors**
Duncan

Posts: 13
Joined: 05 May 2005

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